Answer: event horizon
The event horizon is the surface of a black hole, it is the border of space-time in which the events on one side of it can not affect an observer on the other side.
That is, at this border also called "point of no return", nothing can escape (not even light) and no event that occurs within it can be seen from outside.
In this sense, on April, 10th 2019 this was demonstrated with the publication of the image, for the first time, of the event horizon of the black hole (which appeared as an orange glow ) in the M87 galaxy. This is the first time in human history a picture of a black hole is taken.